The present invention relates to piezoelectric sensors of the type which are used to sense movement or vibration. More specifically, the present invention relates to diagnostics of such piezoelectric sensors.
Piezoelectric sensors are used to sense movement or vibrations in many applications. A piezoelectric sensor comprises a piezoelectric crystal which is typically mechanically coupled to an object which produces a mechanical movement. This causes a mechanical input to the crystal which results in an electrical signal being generated across the crystal. By measuring the electrical signal, it is possible to make determinations regarding the mechanical input which was applied to the crystal.
One application for piezoelectric sensors is in vortex flowmeters. Vortex flowmeters are capable of measuring flow of a gas or fluid (referred to as xe2x80x9cprocess fluidxe2x80x9d) by monitoring the vibration generated by a bluff body as the fluid flows past the buff body. The piezoelectric sensor is mechanically coupled to the bluff body and generates an electrical output related to the amplitude and frequency of the vibrations in the bluff body. This electrical output can be correlated to the rate at which the gas or fluid is flowing past the bluff body. Such vortex flowmeters are used in industrial processes in which it is desirable to monitor the flow rate of a process fluid.
Failure or degradation of the piezoelectric sensor can cause inaccuracies in measurements obtained from the sensor. If a sensor has wholly or partially failed yet remains in use, the measurements generated from that sensor will be erroneous. In order to ensure that a sensor has not failed it can be necessary to periodically test the sensor. Even in situations where it is clear that the measurements are erroneous, it is still necessary to conduct tests on the sensor and measurement electronics to determine the source of the error. Such testing typically requires that the piezoelectric sensor be taken to a laboratory or placed in some sort of test fixture. This can be time consuming. In industrial process environments in which a vortex flowmeter is located at a remote location, and testing requires temporarily shutting down the process, the testing procedure can be particularly cumbersome.
A diagnostic device for testing a piezoelectric sensor includes an AC source configured to apply an AC signal to the piezoelectric sensor at two or more different frequencies. Measurement circuitry coupled to the piezoelectric sensor measures a response of the sensor to the applied AC signal and provides a measured output related to a sensor resistance and a sensor capacitance of the piezoelectric sensor. Diagnostic circuitry provides a diagnostic output as a function of the measured output.